It stroked the stones and spoke in tongues and splashed against my face,

Till ages rolled, the sun shone cold on this unholy place.

That was the planet Mars as channeled by the folk singer and science writer Jonathan Eberhart in “Lament for a Red Planet.”

Ever since the Italian astronomer Giovanni Schiaparelli thought he spied lines that he called “canali” on Mars in 1877, earthlings’ romantic thoughts about our nearest cosmic neighbor have revolved around water and its possible consequence, Life as We Know It. We haven’t found life on Mars, but decades of robotic exploration have indeed strengthened astronomers’ convictions that rivers and perhaps even oceans once flowed on the red planet.

Today Mars is an arid, frigid desert, suggesting that the mother of all climate changes happened there, about four billion years ago or so. The question that haunts planetary scientists is why? And could it happen here?

“I think the short story is the atmosphere went away and the oceans froze but are still there, locked up in subsurface ice,” said Chris McKay, an astrobiologist and Mars expert at NASA’s Ames Research Center.

In September a new spacecraft known as Maven, the Mars Atmosphere and Volatile Evolution mission, swung into orbit around the planet. Its job is to get a longer answer to one part of the mysterious Martian climate change, namely where the planet’s atmosphere went.

One idea is that it was sputtered away by radiation and particles from the sun, known as the solar wind. Maven was designed to test that theory by measuring how fast Mars is losing atmosphere today. The results could help scientists determine what the atmosphere was like four billion years ago, and just how warm and wet the planet was.

“We’re going to get some suggestive answers,” said Bruce Jakosky, a University of Colorado professor and principal investigator for Maven.

The results could resonate beyond Mars or even our solar system, shedding light on the fickle habitability of exoplanets. Alien astronomers looking at our solar system with a good telescope four billion years ago might have concluded that Mars was a likely habitat for life. Now look at it.

“What we are learning about are planetary atmospheres in general,” said David Brain, a Colorado astronomer and Maven team member. “It’s really fascinating to think that the planet changed in such a large way.”

Everybody agrees that Mars was once wetter, on the basis of two lines of evidence. The surface of the planet is crossed with features that resemble old river channels, like the tributaries and canyons that lead into Chryse Planitia, the Plain of Gold, an ancient crater 1,000 miles wide and a mile-and-a-half deep. And NASA’s rovers have found minerals characteristic of watery environments, formed four billion years ago.

But answers on exactly how wet and warm Mars was — and for how long — depend on whom you talk to.

According to one camp, Mars back then had a thick atmosphere with enough carbon dioxide, the greenhouse gas looming big in Earth’s future, to warm up the temperature and keep it there for the hundreds of millions of years it would have taken to carve the Martian river system. Others have suggested that phenomena like asteroid impacts or the tilting of Mars’s poles could have produced shorter periods of near-freezing temperatures. The impact that created the huge crater called the Hellas basin, for example, would have hurled vast amounts of vaporized rock into the sky — leading to decades or centuries of hot rain and flash floods, said Brian Toon of the University of Colorado. It might have been followed perhaps by a lingering era of nearly freezing temperatures as clouds left over from the steam bath produced a mild greenhouse effect.

Some geologists question whether the complicated river systems on Mars could have been created in such relatively short episodes, but they admit a serious flaw in their alternative view of a long-lasting greenhouse atmosphere of carbon dioxide. Namely, where did it go?

“The holy grail of Mars,” said Dr. Jakosky of the Maven team, is to find the carbonate deposits that should have formed from its atmosphere. “We haven’t found them,” he said.

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Exploring Mars

James B. Garvin, a chief scientist with NASA, discusses the history of Mars exploration, current technological advances and what is still to come in understanding our planetary neighbor.

Which is where the new Maven mission comes in. One of the most striking clues that something has happened on Mars has come from atmospheric measurements from previous probes. They have shown that the lighter forms, or isotopes, of elements like hydrogen, nitrogen and argon are strangely depleted by contrast with their abundance on Earth.

On Mars the ratio of heavy nitrogen, which has an extra neutron in its nucleus, to regular nitrogen is twice that of Earth. The same pattern goes for argon, which is Dr. Jakosky’s favorite because it is chemically inert and can’t disappear from the inventory except by being swept out of the atmosphere.

All told, Dr. Jakosky said, the isotopic ratios on Mars suggest that about 60 to 90 percent of the atoms that were once in the Martian atmosphere might have been lost to space.

“We know the mechanism by which it was lost, but we can’t quantify it yet,” he said.

The story goes something like this. Once upon a time, Mars had a magnetic field that, like Earth’s, acted as an umbrella, deflecting the rain of energetic particles shed by the sun. Earth’s field is generated by a dynamo, which in turn is powered by rising heat, convection in the planet’s molten iron core. Once Mars cooled off, the dynamo and the magnetic field stopped and the solar wind began pecking away at Mars’s atmosphere. Ultraviolet radiation from the sun would ionize atoms in the upper atmosphere, making them subject to forces from magnetic fields carried along in the solar wind, and they would slip away into space an atom or two at a time.

“A little bit every few hours,” Dr. Brain of Colorado said, and “suddenly you can change an entire planet.”

Eventually — with no atmosphere, no rain and none of the tectonic churning that keeps Earth’s oceans refreshed — the Martian rivers and oceans, if any, would have been absorbed into the ground and frozen, said James Kasting, a geoscientist at Penn State. Indeed, orbiting spectrometers have detected the signature of water in the form of ice under the wasted and lonely red sands.

In September, after a 10-month trip from Earth and just in time to observe the effects of Comet Siding Spring pass by Mars, Maven began settling into a looping orbit around Mars, flying as close as 77 miles. Its instruments will observe the sun and solar wind; Mars’s upper atmosphere, the pool from which escaping particles are drawn; and the particles themselves as they escape. By understanding how the atmosphere is reacting to the sun today, Dr. Jakosky said, scientists should be able to extrapolate and say how much of the Martian atmosphere has been removed to space over the eons.

If the amount lost is substantial — “a couple of bars of CO2,” he said, describing it in units of the atmospheric pressure on Earth — “would tell us that Mars must have been warmer in the past.”

If losses are trivial, he said, that would spell death for the early greenhouse theory, and the great Martian arguments would continue.